[jkarneges]

Following #48459 which targeted headers, this PR replaces as many uses of QByteArray with CowByteArray as possible in the rest of the core module, without removing compatibility code.

With this, all use of QByteArray is eliminated from the core module except:

• Where necessary to interoperate with other Qt APIs, e.g. calls to .asQByteArray(). These uses will go away as we migrate away from these other APIs.
• Its use in the backing implementation of CowByteArray. This use will go away when it is the only usage remaining and we swap it out for a non-Qt implementation.

% rg --pcre2 -g 'src/core/*' -g '!cowbytearray*' -i '(?<!as)qbytearray'
src/core/qtcompat.h
41:    ByteArray = QMetaType::QByteArray,

src/core/cowurl.h
22:#include <QByteArrayView>
95:    return qHash(QByteArrayView(s.data, static_cast<qsizetype>(s.len)), seed);

src/core/statusreasons.cpp
39:    return CowByteArray(QByteArray::fromRawData(s, size));